This invention relates to correction of the output of a universal exhaust gas oxygen sensor which detects an oxygen concentration in exhaust gas of an internal combustion engine.
In order to maximize the performance of a three-way catalyst which removes toxic components such as hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx) from engine exhaust gas, it is necessary to maintain an oxygen concentration of the gaseous environment of the catalyst equal to that of the burnt gas produced by the combustion of fuel mixture of a air-fuel ratio.
The catalyst has the function of storing and releasing oxygen in response to the oxygen concentration in a catalytic converter storing the catalyst such that the gaseous environment of the catalyst is maintained at an oxygen concentration corresponding to the stoichiometric air-fuel ratio.
In order to maximize the oxygen storage/release function of the catalyst, it is desirable that a target value for the catalyst oxygen storage amount is set to half the oxygen storage capacity of the catalyst and that the air-fuel ratio of the fuel mixture supplied to the engine is controlled to maintain the oxygen storage amount of the catalyst to the target value.
U.S. Pat. No. 5,842,340 discloses a calculating method of the oxygen storage amount of the catalyst. This method estimates the oxygen storage amount of the catalyst by analysis of an output signal of oxygen sensors provided upstream and downstream of the catalytic converter.
It also discloses control of the air-fuel ratio of the fuel mixture supplied to the engine so that the oxygen storage amount coincides with the target value.
A similar method is also disclosed in Tokkai Hei 5-195842 published by the Japanese Patent Office in 1993 and Tokkai Hei 7-259602 published by the Japanese Patent Office in 1995.
The above method uses a universal exhaust gas oxygen sensor which can detect a wide range of oxygen concentrations for the oxygen sensor provided upstream of the catalytic converter. The universal exhaust gas oxygen sensor has a tendency to deteriorate overtime due to exposure to high exhaust gas temperatures. Furthermore errors may result in detected oxygen concentrations due to quality control problems during manufacture of the sensor.
Such deterioration or manufacturing errors result in reductions in the calculation accuracy of the oxygen storage amount of the three-way catalyst that may result in a reduction of the exhaust gas purification performance of the catalyst.
It is therefore an object of this invention to correct fluctuations in the output of the universal exhaust gas oxygen sensor provided upstream of the catalyst due to the deterioration or manufacturing errors and to increase the calculation accuracy of the oxygen storage amount of the catalyst.
In order to achieve the above object, this invention provides an air-fuel ratio controller for such an engine that comprises an exhaust passage, a catalytic converter disposed in the exhaust passage to purify exhaust gas, the catalytic converter accommodating a catalyst which stores oxygen when an oxygen concentration in exhaust gas is higher than a predetermined concentration and releases oxygen when the oxygen concentration in exhaust gas is lower than the predetermined concentration, and a fuel injector which supplies fuel to the engine.
The controller comprises a first oxygen sensor which detects an oxygen concentration in the exhaust passage upstream of the catalytic converter and outputting a corresponding signal, a second oxygen sensor which detects an oxygen concentration in the exhaust passage downstream of the catalytic converter and outputting a corresponding signal, and a microprocessor.
The microprocessor is programmed to calculate a fuel injection amount of the fuel injector to cause an output signal of the first oxygen sensor to coincide with a value corresponding to the stoichiometric air-fuel ratio, calculate an oxygen storage amount of the catalyst based on the output signal of the first oxygen sensor, correct a fuel injection amount to cause the oxygen storage amount to coincide with a predetermined target value, and control the fuel injector to inject a corrected fuel injection amount.
The microprocessor is further programmed to determine if an output signal of the second oxygen sensor is fluctuating periodically between a stoichiometric region and a specific region outside the stoichiometric region. Herein, the stoichiometric region is defined as a region about the value corresponding to the stoichiometric air-fuel ratio. The microprocessor is further programmed to accumulate, when the output signal of the second oxygen sensor is fluctuating periodically between the stoichiometric region and the specific region, an excess/deficiency oxygen amount of exhaust gas flowing into the converter based on the output signal of the first oxygen sensor, and correct the output signal of the first oxygen sensor based on an accumulated excess/deficiency oxygen amount.
The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings.